1. Field of the Invention
The present invention relates generally to a liquid ejection printing apparatus and a liquid supply method to be employed in the same. More particularly, the invention relates to a liquid ejection printing apparatus in a technical field of an ink-jet printing, in which a large amount of ink is used and a circulation path of the ink is provided, and a liquid supply method employed in the liquid ejection printing apparatus.
2. Description of the Related Art
Conventionally, an ink-jet printing apparatus performing printing on a medium to be printed, such as a paper by ejecting an ink droplet from a printing head, has been spread as a printer connected with personal computer, a facsimile machine, a copy machine.
The ink-jet printing apparatus is advantageous for capability of high speed and low noise printing of high quality image, and for easiness of color printing. In the recent years, the ink-jet printing apparatus is also used for apparatus having large printing volume, such as printing on a large size paper, such as poster output, textile printing performing printing on a cloth, and so on.
On the other hand, study has been made for application of the ink-jet printing to a multi-tone image printing for medical use, such as X-ray film, CT-scan image and so on. In such medical image printing apparatus, it becomes possible to express much greater variety of tones than number of kinds of ink by combining a plurality of kinds of inks having different densities and performing printing for two or three times.
Here, since ink-jet printing apparatus performs printing by ejecting inks, it becomes necessary to constantly supply ink consumed by ejection.
As an ink supply system for the printing head, in case of the known apparatus for respective application consuming large amount of ink, a large volume tank is integrally mounted on a main body in the apparatus and an ink passage from the tank to the head cartridge is formed with a pipe, such as a tube.
As a flexible tube to be employed in the ink passage, a tube formed of rubber or resin has certain gas permeability. Therefore, air in the atmosphere may penetrate into the tube through a tube wall in bit by bit to cause bubble therein. When the bubble flows into the printing head, normal ejection of the ink droplet becomes impossible to cause printing failure in the worst case.
Therefore, there has been proposed an ink circulation passage as shown in FIG. 1 which prevents the bubble from entering into the printing head even when the bubble is generated.
In FIG. 1, the reference numeral 501 denotes a printing head, 502 denotes an air buffer chamber, 503 denotes an ink tank. The air buffer chamber 502 and the ink tank 503 are connected through two ink passages 504 and 505 to form a circulation passage. In a flow path of the ink passage 505, a pump 506 is provided to transfer the ink within the air buffer chamber 502 to the ink tank 503. On the other hand, the air buffer chamber 502 is enclosed structure with respect to atmosphere except for the two ink passages. To the ink tank 503, an atmosphere communication opening 507 is provided.
In the foregoing ink circulation passage, when the pump is driven, the ink and air within the air buffer chamber 502 is transferred into the ink tank 503 through the ink passage 505. In conjunction therewith, the ink in the ink tank 503 is fed to the air buffer chamber 502 through the ink passage 504. By performing ink circulation between the air buffer chamber 502 and the ink tank 503, bubble generated in the ink passage can be discharge into the ink tank 503 to successfully prevent the bubble from penetrating into the head.
However, in the foregoing construction, the following technical problems have been encountered in view point for performing further printing of higher tone and higher image quality image at higher speed.
Namely, when the pump 506 is provided within the ink circulation passage, the ink passes through the inside of the pump to cause difficulty of certainly removing dust generated within the pump (for example, in case of a tube pump, worn chip of the inside of the tube caused due to repeated drawing of the same position of the tube or fat and oil component contained in the material of the tube). Such dust can cause plugging if deposited on the nozzle in the printing to be solidified therein. Particularly, when the diameter of the ejection openings or nozzles are made smaller in order to realize high image quality, certain removal of the dust is required.
In commonly owned Japanese Patent Application Laid-open No. 6521/1998, there has been proposed a liquid supply method and a light supply apparatus for easily and certainly perform supply of the liquid into the liquid supply path with solving the problem in removing dust in the ink supply passage in one direction, by providing a mechanism for feeding the ink independent of the ink supply passage.